This invention relates to slit lamps of the type incorporated in instruments used to illuminate eyes for ophthalmic examination and/or photography; and, more particularly, it relates to controlling the illumination produced by such slit lamps.
In such instruments, the eye being examined is illuminated from the side by an angularly adjustable slit lamp. The illumination emerges from a narrow end surface of the lamp's illuminating prism and is normally diffused to increase the area of the illumination impinging on the target. In prior art devices, such diffusion has been accomplished by means of a ground glass platelet placed over the surface of the prism edge from which the illumination emerges. The roughened surfaces of such prior art platelets are produced by grinding with an abrasive, or by etching, or by grinding followed by etching.
As is well-known in the art, the grain size of the abrasive, or the etching technique, can be varied to determine the resulting granularity (roughness) which is distributed irregularly over the surface of the ground glass. The degree of such roughness, in turn, determines the angles of diffusion and the character of the resulting illumination.
Although such prior art ground glass platelets serve to diffuse the illumination of the slit lamp over larger areas, the random irregularity of the platelet surface results in significant light losses, because the area of illumination does not conform well to the oval area of the eye's palpebral fissure (i.e., the area between the margins of the eyelids when the eye is open). That is, such slit lamp illumination suffers from a vignetting effect and does not adequately illuminate the ends of this oval area. This is particularly noticeable when using prior art slit lamp illumination for purposes of reproducing the palpebral fissure photographically because proper exposure of the central portion of the oval is often accompanied by insufficient exposure of its end portions.